The website Ain't It Cool has put up a hands on preview of GTA IV, Monki (the article writer) give us a very casual look into his time with GTA IV, there isn't any new information but it might be worth a read.

Steve started walking Niko through the city while he spoke to me about various features in the game...more music than ever...lots of Eastern European music that hasn't been released in America...very cool stuff. But perhaps the coolest thing I saw while he was playing was when he stopped touching the controller. Niko stood still while Steve was occupied with something and right there, a taxi got rear-ended by another car.
What followed was pure magic.

The cabbie and the guy who hit the taxi jumped out of their cars and approached each other. Once they got in to each others' face the fists started flying. Meanwhile, cars are honking for the two vehicles to get out of the way. The two keep slugging away and a cop pulls up. He jumps out with his pistol drawn and both men throw their hands to the sky, surrendering. The cop then drags the cabbie away (he punched first), puts him in the backseat of the cop car and drives off. The other driver returns to his car, gets in and pulls away.